RF-lightwave links are commonly used to carry RF signals on optical fiber over long distances with less interference than conventional RF cables or waveguides. In order to carry the RF signals over optical fiber, an optical modulator is used to modulate a lightwave with the desired RF signal to be carried. The usefulness of RF-lightwave links depends largely on the net signal gain or loss of the link. One technique for improving the gain is to provide an amplifier in the link. However, amplifiers are active elements that introduce noise into the system. Another approach is to provide an impedance matching circuit directly between the RF signal and the optical modulator. However, using an impedance matching circuit generally only improves the gain by about 3 dB.
Another passive approach for providing improved gain involves using a modulator electrode that is also a RF resonator. However, in such a technique the lightwave is carried by an optical waveguide which is part of an optical modulator that is coupled to many points along the length of the resonator. As a result, the voltage driving the modulation is the average voltage over the entire resonator rather than the peak voltages found at specific locations on the resonator. Such in-line resonators are discussed in:
1. Digest 1997 IEEE, Topical Symposium on Millimeter Waves, pp. 125–128 (1998).
Previous in-line RF resonators that act as modulator electrodes to drive optical modulators are discussed in:
2. Electronics Letters, vol. 37, no. 20, pp. 1244–1246 (2001);
3. IEICE Trans. Electron., vol. E85-C, no. 1, pp. 150–155 (2002);
4. J. Lightwave Technology, vol. 19, no. 9, pp. 1287–1297 (2001).
These in-line transmission-line resonators were typically T-shaped structures. In such resonators, the top of the T is a resonator that would receive a RF input signal through the leg of the T. The entire top of the T comprises the modulation electrode. The total length of the top of the T defines the effective length of the resonator, which in turn determines the modulation voltage applied to the lightwave in the optical modulator. The total length of the T-top also determines the resonance frequency of the resonator, and thus the passband frequency of the optical modulator. When the T-shaped resonator is driven by the RF input signal, a standing-wave voltage pattern is established across the T. The optical modulator whose optical waveguide is electrically coupled to the entire length of the T-top effectively receives an equivalent of the standing-wave voltage pattern across the top of the T. If the ends of the T-top are electrically short circuited to a ground plane, the maximum voltage in the standing-wave voltage pattern will appear mid-way between the ends of the T-top and the leg of the T. If the ends of the T-top are an electrically open circuit with respect to the ground plane, then the maximum voltage in the standing-wave voltage pattern will appear at the ends of the T-top. In either situation, the standing-wave voltage pattern across the T-top is not uniform. The non-uniform voltage pattern across the T means that the average or cumulative voltage driving the optical modulator is lower than the maximum voltage on the T in the standing-wave voltage pattern.
There have been attempts to improve the voltage distribution across the T. In these attempts, various portions of the T were bent so that a smaller portion of the T-top is coupled to the optical waveguide to be modulated and acts as the modulation electrode of the modulator. This improved the cumulative or average voltage across the region that was being used to drive the optical modulator. For example, if the T has open circuits at the ends of the T-top, the ends of the T-top were bent in a direction orthogonal to the T-top so that only the unbent portion of the T-top was coupled to the optical waveguide to be modulated. For a T whose ends are short-circuited, the lower portion of the leg of the T would be bent orthogonally to the remaining portion of the T-leg. Thus, the portions of the T-top near its ends would be coupled to the optical waveguide to be modulated. These techniques would help decrease the area of the resonator having low standing-wave voltage that is coupled to the optical waveguide to be modulated, thereby helping to increase the average voltage being used to drive the optical modulator. The gain in voltage using these techniques was around 10–12 dB. Nevertheless, the standing-wave voltage pattern of the portion of the T-top driving the optical modulator was still substantially non-uniform. In these and the previous modulators the resonator is in-line with the propagation direction of the optical waveguide. The lightwave in the optical waveguide is thus coupled to many points along the length of the selected portion of the resonator. As a result, these techniques were not able to drive the optical modulator with the maximum voltage found in the standing-wave voltage pattern across the T.
Other types of resonators have also been used in the RF industry but have not been applied to optical modulators. Broken loop resonators have been utilized in the RF industry, but only for use as a RF filter. These resonators were used to reduce the resonant frequency by placing a capacitive load across the gap, but not as a drive source for a modulator. Such a technique is discussed in:
5. IEEE Trans. Microwave Theory and Techniques, vol. 45, no. 12, pp. 2358–2365 (1997); and
6. IEEE Trans. Microwave Theory and Techniques, vol. 37, no. 12, pp. 1991–1997 (1989).